The reaction injection molding process (RIM) has been used to produce reactively processed expanded moldings from the reaction of a polyisocyanate composition with an organic isocyanate reactive composition, in the presence of water as a chemical blowing agent. These expanded moldings are often produced with added reinforcing materials, such as short fibers added to the chemical precursors (a process known as reinforced reaction injection molding, or R-RIM), or relatively long-fiber reinforcing structures such as mats pre-placed within the mold cavity (a process known as low-density structural reaction injection molding, or LD-S-RIM).
Shaped polyurethane-urea resin articles containing long fiber reinforcing structures, such as glass fiber mats, are routinely produced by the well known process called structural reaction injection molding (S-RIM). These structural composite articles are typically foamed in order to reduce part weight, to assist in mold filling, and to minimize part costs. Foamed S-RIM composites are often referred to as low density S-RIM (or LD-S-RIM). Polyurethane-urea S-RIM composite molded articles are prepared by impingement mixing a liquid stream of polyisocyanate with at least one other stream containing active hydrogen-containing liquids and optionally catalysts, fillers, mold release agents, etc. This reacting mixture is then transferred to a heated metal mold. A glass mat or a mat of another type of structural reinforcing fibers is placed in the mold prior to the impingement mixing of the liquid components. The reacting mixture penetrates the fibrous reinforcing mat and cures to form a shaped reinforced composite molded part. When one of the reactive liquid chemical precursors (components) contains a foaming agent, a foamed S-RIM part (LD-S-RIM article) is obtained. Water is the most commonly used foaming agent in polyurethane-urea LD-S-RIM molding processes, but other blowing agents may be used. The water is typically incorporated into the liquid isocyanate reactive component. When the reactive components are mixed, foaming occurs due to the formation of carbon dioxide formed from the reaction of the polyisocyanate with water. Urea linkages are also formed as a consequence of the isocyanate and water reaction. These urea linkages contribute to the buildup of molecular weight and eventual curing of the polymer. Urethane linkages are formed from the reaction of the polyisocyanate with organic polyols present in the isocyanate reactive component. Reactive chemical formulations used for the production of polyurethane-urea S-RIM (and LD-S-RIM) composite parts typically consist of two components; a polyisocyanate component (or A-component) and a single isocyanate reactive component (or B-component). The B-component most commonly comprises a mixture of the organic polyols with water as foam blowing agent. Water is typically the sole blowing agent. The B-component typically also contains the optional additives, such as catalysts and other optional ingredients. Catalysts, although optional, are typically used in these formulations in order to obtain an economically acceptable cure rate. S-RIM and LD-S-RIM processes are commonly used to manufacture shaped composite automotive articles such as interior trim parts, door panels, package trays, speaker enclosures, seat pans, and the like.
Developments in the chemistry of polymer systems used in S-RIM processes have resulted in urethane and urethane-urea polymer systems which are sufficiently cured to be demolded within about 65 to 105 seconds, as measured from the time that the injection of the reacting liquid component mixture into the mold has been completed. S-RIM processing equipment has also improved so that the mechanics of opening and closing the mold also require only about 10 to 70 seconds. Isocyanate-based polymers are excellent adhesives that bond tenaciously to surfaces such as decorative facings, such as vinyl or cloth, as well as to reinforcing fibers. This facilitates the consolidation of parts in the manufacture of very complex composite articles by means of S-RIM (including LD-S-RIM) processes.
Problems with unwanted adhesion of S-RIM moldings to the mold surface, which can result in difficulties in removing molded parts from the mold or damage to the parts during the process of demolding, have been addressed through the development of mold release technology. External mold release agents are used by applying a release agent directly onto the surface of the mold, generally by spraying or wiping. More recently internal mold release technologies have been developed, which greatly increase the number of consecutive parts that can be molded from a single application of an external release coating. Internal mold releases are chemical additive packages which are incorporated into one or more of the chemical precursor components of the S-RIM article, usually the B-component. A description of a particularly effective class of internal mold release agents, and the application thereof in S-RIM technology, is provided in, for example, U.S. Pat. Nos. 5,576,409 and 5,670,553. Combinations of state of the art internal and external mold release technologies have greatly reduced the cycle time per molded part in large-scale S-RIM production operations. These technologies have made it possible to mold hundreds of parts in succession without the necessity of cleaning the mold and re-applying the external mold release coating. This has considerably improved the productivity of the S-RIM process.
In spite of many recent improvements to the production economics of S-RIM processes, there is a strong need for further improvements in the direction of reduced cycle time per molded part. This need is strongest in the LD-S-RIM area, where foaming places severe practical limits on the minimum mold residence time per part.
Foaming of S-RIM composites is extremely important for weight reduction and cost minimization in automotive applications. The foaming reaction in typical water blown polyurethane-urea LD-S-RIM processes yields a molded composite part with a cellular structure. The foaming process currently dictates the minimum mold residence time for these parts. Even when the molded part has cured to the point where it is strong enough to be demolded without damage, the liberation of gas from the polymer can cause the part to swell and/or crack after the mold has been opened and even after the part has been completely removed from the mold. The swelling and/or cracking render the part unsuitable for use. This process, known commonly as “post blow”, can result in grotesque internal splits and/or part swelling. The problem is most severe in thick parts, or thick sections of parts. Post blow can occur whether or not the expanded molded part contains reinforcing materials. The causes of post blow are not completely understood. It has been theorized that post blow may be caused by a continuation of the isocyanate and water reaction even after the part has developed adequate “green strength”. Another possibility is that some of the blowing gas (CO2) does not result in the formation of cells, but is instead dissolved in the polymer phase. According to this conjecture the dissolved gas comes out of solution when the mold pressure is released, resulting in splits and voids unless the polymer has reached a very advanced state of cure. Yet another theory suggests that the post blow phenomena are caused by the presence of large numbers of closed cells in the molded part, and hot gas due to the reaction after-heat. The true cause of post blow may be a combination of these things, or something else altogether. The important thing is that post blow phenomena put a practical lower limit on the minimum time that the part must be left in the mold before it can be safely removed (without post blow damage). This practical lower limit varies with the size and the geometry of the part but is typically about 105 seconds, as measured from the time that the injection of the reacting liquid component mixture into the mold has been completed.
In multi-part production runs, the percentage of scrap parts (i.e. parts with defects caused by post blow) increases sharply as the mold residence time is reduced below about 105 seconds. As the mold residence time is reduced below about 105 seconds, using the polyurethane urea LD-S-RIM systems of the prior art, a majority of the parts are of unacceptable quality (scrap). As the mold residence time is further reduced to below about 85 seconds, essentially all the moldings are scrap. Although the precise relationship between scrap rate and mold residence time varies somewhat with part geometry, it generally holds for geometrically complex parts, such as interior door panels, which are commonly produced in large volumes by the LD-S-RIM process in the automotive industry. A high scrap rate is clearly undesirable for the economics of the process, and also for environmental reasons. In large scale molding operations, common in the automotive industry, a scrap rate of greater than 5% would be unacceptable regardless of how short the mold residence time. A scrap rate of less than 1% is generally considered acceptable. A scrap rate of less than 0.5% is more desirable. In general, a formulation or process that can offer a reduction in mold residence time (and hence a reduced overall cycle time per molded part) along with a constant or reduced scrap rate would be considered highly desirable in industry.
The post mold expansion (post blow) problem described above causes defects in part quality that cause scrap rates to go up quickly as mold residence times are reduced. These defects include internal splits and/or cracking that are visible at the surface of the part. Splits are especially problematic in thick parts or thick sections of parts. The splits may not be visible on the part surface. A tell tale sign that splitting has occurred is a visible bulge in the part upon mold opening that does not go away. This will result in a part being scrapped. Obviously, a split that is directly visible on the surface will also result in the part being scrapped.
Another important class of defects are large voids and bubbles that are visible on the top surface of a part, especially those which form immediately beneath a facing material (such as vinyl coverstock). These large surface (or near-surface) bubbles can be seen in the surface of the part, and make it cosmetically unacceptable. These kinds of surface-visible defects are important in “pour behind” molding processes (wherein the liquid reaction system is injected or poured behind the facing material in the mold). Although the presence of such large surface-visible defects (sometimes referred to as voids; blisters; or as de-lamination, in the case of very large bubbles behind impervious facings) can sometimes be fixed (i.e., by puncturing the facing above the bubble or filling the area of the void where necessary), these large visible defects usually result in scrap parts.
In the industry the term “void” generally refers to a large hole or bubble within the foam itself, whereas the term “bubble” refers to an area of interfacial non-adhesion between the foam part and a facing material thereon (such as a vinyl facing layer). If the area of interfacial non-adhesion (bubble) is large enough it will become quite an obvious defect, since trapped gases within the part will collect in this non-adhering interfacial area and raise a large bubble under the facing material. Voids, on the other hand, may begin as nucleation points in the foam itself which subsequently grow and/or coalesce as the reaction mixture expands (resulting in a bulge, or a depression on the surface, which indicates the presence of a void in the foam).
The presence of a surface-visible void or bubble of greater than an inch across (at its widest point) will result in a molded part being scrapped. In fact, a surface-visible void or bubble of greater than one half inch across will usually result in a part being discarded as scrap. It is therefore desirable to have formulations or processes that can be used to produce molded parts with reduced mold residence time, but without increased tendency to form splits or large surface-visible voids or bubbles (surface-visible defects).
The reaction chemistry, the processing equipment, and the use of internal mold release technology have made it theoretically possible to achieve minimum mold residence times for LD-S-RIM parts of much less than 105 seconds (as measured from the time that the injection of the reacting liquid component mixture into the mold has been completed). It is in fact possible in principle to demold such parts in less than 65 seconds, and even less than 55 seconds. A doubling of the productivity of LD-S-RIM could therefore be achieved if these very short mold residence times could be achieved in practice, without increasing the scrap rate. This dramatic further improvement is currently not possible, because of defects caused by post blow phenomena. It would be of considerable value to develop an LD-S-RIM system with reduced processing cycle times, which does not suffer from the limitations imposed by post blow problems, but which otherwise performs at the level of the current state of the art using existing processing equipment and without requiring special processing conditions. It would be desirable to have water blown LD-S-RIM systems from which viable parts can be demolded in less than 65 seconds, as measured from the point that the injection of the reacting liquid component mixture into the mold has been completed. It would further be desirable to have other types of water blown RIM processable reaction systems from which viable parts can be demolded in under 65 seconds, whether or not a reinforcing material is used.